This invention relates to pneumatic controls, and more particularly its relates to a device for controlling the rate of flow of breathing gas supplied to a patient by a medical lung ventilator.
It is of great importance in respiratory therapy carefully to control the gas flows to the patient to avoid injury and to insure that a maximum therapeutic effect is achieved. To this end, various flow control valves are normally installed in the conduit system which connects the ventilator air supply with the patient. One of these valves is typically a peak flow control valve which restricts the maximum volumetric rate of flow of gas to the patient to a predetermined amount for optimal therapeutic effect. The peak flow rate is normally reached during an early stage of inspiration, following by a gradually decreasing intake of air, until the patient is ready to exhale. Another valve is frequently employed to control the rate in which the inspiratory air flow tapers off, with the optimum decay rate, or deceleration, depending upon the type of therapy administered.
Various devices have been employed in the medical ventilator art to provide a controlled airflow decay rate or "taper" rate. Examples of such devices are disclosed in U.S. Pat. No. 3,756,229 to Ollivier and U.S. Pat. No. 4,177,830 to Munson.
Many practitioners in the field of respiratory therapy deem it advisable, with certain patients, to provide the patient with a substantially constant flow rate throughout the inspiratory part of the respiratory cycle. Accordingly, such practitioners require a ventilator which is capable of delivering what is known as a "square wave" airflow, in which the flow rate is maintained at or near the peak flow rate for all or part of the inspiratory period. Accordingly, the ventilator art has sought the development of ventilator devices having both square wave and taper wave capabilities. One approach to this problem is disclosed in the aforementioned patent to Munson. The Munson patent discloses a valve assembly which allows the decay rate to be adjusted without adjusting the initial peak flow rate. The decay rate, or taper, can be adjusted to select a minimum value thereof so as to approximate a square wave type of flow. However, at high peak flow rates, even this minimal amount of taper can be substantial. For example, at peak flow rates of 100 liters per minute (LPM), the "square wave" setting of the valve still allows approximately a 20% taper or decay in the rate of flow.
From the foregoing, it will be appreciated that a medical ventilator which is capable of providing close to true square wave performance, as well as a controllable tapered flow, would be a significant improvement over the prior art.